The use of drilling fluids for the drilling of subterranean boreholes is well known. The drilling fluid serves numerous purposes, including, for example, suppression of formation pressure, lubrication of the drill string, flushing drill cuttings away from the drill bit, cooling of the bottom hole assembly, driving turbines that provide power for various downhole tools, and powering downhole progressive cavity motors. In use drilling fluids are typically pumped down through the tubular drill string to the drill bit and circulate back to the surface in the annular region between the drill string and the borehole wall. The circulating drilling fluid typically carries drill cuttings, metal shavings, and other debris to the surface. Large particles, having a size that may damage sensitive downhole tools, such as various measurement while drilling (MWD) or logging while drilling (LWD) tools, or plug drill bit jets are desirably removed from the drilling fluid before recycling back into the borehole.
Various surface filtering techniques are well known in the industry for removing drill cuttings and other debris from the drilling fluid. For example, shaker tables are commonly used to screen out relatively large particles (e.g., having a diameter greater than ⅛ inch). Centrifugal tools, such as desanders and desilters are also commonly used to remove abrasive solids prior to recycling the drilling fluid back into the borehole. However, it is not uncommon for such surface filtering techniques to fail, resulting in large drill cuttings and debris being pumped downhole. Additionally, various “foreign objects”, such as tools, rags, gravel, chunks of plastic from thread protectors, and the like are sometimes introduced into the drilling fluid through human error and inadvertently pumped downhole.
As a redundant measure, pipe screens are commonly used on the topmost section of drill string with the intention of preventing large particles and debris from being pumped downhole. While such pipe screens have been successfully utilized and are commercially available, they are nevertheless prone to failure in that operator intervention is required to remove, clean, and reposition the screen each time a new length of drill string is added. Furthermore, damaging scale and/or cement particles often originate from locations within the drill string. Scale particulate may result, for example, from corrosion of the drill string components or various mineral deposits, while cement particles are sometimes deposited on the interior of the drill string during cementing operations. Such particles are sometimes freed during drilling operations and are a known source of blockage or damage to downhole tools.
In an attempt to overcome such difficulties, retrievable downhole filtering tools are known, for example, those disclosed by Beimgraben in U.S. Pat. No. 4,495,073, Taylor in U.S. Pat. No. 6,296,055, and Mashburn in U.S. Pat. No. 6,598,685. Such retrievable filtering tools are intended to be periodically removed from the drill string and cleaned (e.g., when the pressure at the mud pump reaches some predetermined threshold). While such prior art filtering tools may, in certain applications, remove damaging particles from the drilling fluid, their retrieval from the drill string is often problematic. For example, in certain drilling applications, it may be advantageous for various sections of the drill string to include a reduced inner diameter. However, such a reduced inner diameter may render it impossible to retrieve the filtering tools. Furthermore, in deep well applications (e.g., at measured depths greater than 10,000 feet), it is sometimes difficult to generate the impact required to dislodge the filtering tool from the drill string (e.g., to shear one or more shear pins). In such instances it is often necessary to remove at least a portion of the drill string from the borehole (at significant expense and time loss) in order to retrieve the filtering tool. Moreover, the act of retrieving such retrievable filtering tools has been known to cause debris to be freed or dumped in the drill string.
Therefore, there exists a need for an improved downhole filtering tool for filtering a drilling fluid. In particular there exists a need for a downhole filtering tool that does not generally require retrieval from the drill string.